Can we please move the RAID discussion off-list? (was RE: filmscanners: Best solution for HD and images)

["Stuart Nixon" <sns@ermapper.com>]

Can we PLEASE take this RAID discussion off-list?

It is not directly related to scanners. And there is enough misinformation
being thrown around here that it is just confusing everyone.

There is plenty of reference information for RAID systems on the web and
elsewhere; we don't need to clutter the list up with this IMHO.

If people want reference information on RAID systems, such as the fact
that RAID 0 is indeed less reliable than a single drive or RAID 1 or 5, I
refer
you to information such as:
http://www.pcguide.com/ref/hdd/perf/raid/concepts/relRel-c.html
http://www.usbyte.com/common/raid_systems_3.htm#Extended%20Data%20Availabili
ty
http://204.56.132.222/courses/CIS312J/FAQ/raid-faq.txt
http://www.dansdata.com/raid.htm
http://www.csr.city.ac.uk/people/lorenzo.strigini/A701/A701material/lecture8
/A701.8.FTnotes_010312A.pdf
http://www.sas.com/partners/directory/sun/wp/raid.txt

Thanks

Stuart

p.s. I saw some Mac users were asking about IDE RAID systems.
Have a look at the new IDE/SCSI RAID 5 boxes from Promise and others,
which have IDE drives, and SCSI out.
http://www.promise.com/Products/UltraTrak/UltraTrak100%20TX4%20&%20TX8%20Dat
a%20Sheet.pdf
A 8 x 100GB IDE drive system gives about 700GB of usable space.  I like
these
external RAID boxes, because they are low cost, have hot swappable drives
and power supplies, and plug straight into a Mac/PC/Unix SCSI controller.


[Original message]

-----Original Message-----
From: owner-filmscanners@halftone.co.uk
[mailto:owner-filmscanners@halftone.co.uk]On Behalf Of Austin Franklin
Sent: Tuesday, 13 November 2001 10:53 PM
To: filmscanners@halftone.co.uk
Subject: RE: filmscanners: Best solution for HD and images


> MTBF of a RAID-0 system (or dual cpu/memory where one unit CAN
> NOT continue
> without the other) will always be lower than a single drive unless the
> standard deviation (they never quote SD) of the MTBF is zero.

Well, if you take duty-cycle into account, which MTBF calculations do, you
will actually get higher MTBF for RAID 0, simply because the main failure is
the servo actuator, and when it is only being used for half the time...MTBF
will increase.

> The reality for MTBF of a RAID-0 will lie in between.

But that means it doesn't change compared to a single drive...

> Cummalative failure rate is a much more useful figure for us and
> for a small
> number of fairly reliable inter-dependant devices this is nearly
> an additive
> figure - but not quite.

That I completely disagree with.  It is absolutely NOT additive.  In fact,
as I pointed out above, you may get HIGHER reliability by using RAID 0
simply because of duty cycle and the common failure mode, both of which are
a very important part of MTBF.

> Seagate reckon about 3.41% (flat-line model) will fail during the first 5
> years of use (assuming you only use it for 2400 hours a year [6
> 1/2 hours a
> day]) :
>
> http://www.seagate.com/docs/pdf/newsinfo/disc/drive_reliability.pdf

If you read that article you referenced, when they talk about multiple
disks, they are talking about multiple PLATTERS in a single disk, not
drives, so you can't derive the numbers you did for multiple drives from
that article.  No where in that article did they discuss multiple drives.